Combination electrodes which are employed in electric arc furnaces for the electric steel production have been known for some time. They comprise an upper metallic section to which a lower section of consumable carbon material is attached by a threaded nipple or a similar means. The two sections may also be directly connected. On account of the high temperatures used, the possibility of arc migration, the existence of splashes of liquid metal and slag as well as other unfavourable influences, it has already been recommended to provide protective coats for the metal shaft. EP-A1 No. 00012573 (British Steel) describes a coat of fireproof material/slag that rests directly on the metal shaft.
GB-PS No. 1,223,162 (Oestberg) describes a cooling cycle made up of metal ducts which are embedded in ceramic material consisting e.g. of crystallized glass materials on a sillimanite basis or of refractory materials containing aluminium oxide. However, this solution is not applicable as far as the practical operation in the electric arc furnace is concerned. Here, the electrodes are frequently exposed to strong mechanical stresses due to vibrations, splashes of liquid metal, and electrode displacement or electrode handling, which will soon result in the damage of the ceramic part.
U.S. Pat. No. 4,145,564 (Andrew et al) describes the use of electrically conductive ceramic materials which rest on the metal shaft in the form of moldings. These ceramic moldings are put on metallic hook elements and are held by metallic spreaders. Specific examples of temperatureresistant ceramic materials are not given. Neither is this electrode suited for the long-term production of electric steel. Exploratory experiments with ceramic rings of refractory aluminates have shown that these protective ceramic elements, without being damaged, are covered with adhesive metal and slag particles which cannot be removed, so that it is no longer possible to extract the electrode throught the opening of the electric arc furnace lid. Lack of safety and high energy losses were the reasons why the extremely high temperatures observed during operation in the metal shaft could not be tolerated any more.